Power modules have been used for controlling heavy-current for electric vehicles, electric railcars, machine tools and so forth. In a conventional power module, a metal/ceramic insulating substrate is fixed to one side of a metal plate or compound material called base plate, and semiconductor chips are fixed to a metal circuit plate of the metal/ceramic insulating substrate by soldering.
In recent years, it is proposed that a silver paste containing fine silver particles is used as a bonding agent to be arranged between articles, such as copper plates, to be heated for a predetermined period of time while pressing the articles against each other, to sinter silver in the bonding agent to bond the articles to each other (see, e.g., Japanese Patent Laid-Open No. 2011-80147). It is attempted that such a bonding agent of a silver paste containing fine silver particles is substituted for solder to fix electronic parts, such as semiconductor chips, on a metal plate of a metal/ceramic insulating substrate.
As such a method for fixing electronic parts on a substrate, there is provided a method for arranging silver nanoparticles, which can be sintered at a low temperature, between the terminal of a semiconductor device and the electrode of the substrate to sinter the silver nanoparticles to bond the terminal of the semiconductor device to the electrode of the substrate (see, e.g., Japanese Patent Laid-Open No. 2007-208082).
There is also provided a method for arranging a metal nanoparticle paste, which contains silver nanoparticles dispersed in an organic solvent, between the silver portion of a semiconductor device and a copper circuit plate on a ceramic insulating substrate to heat the paste to bond the silver portion to the copper circuit plate (see, e.g., Japanese Patent Laid-Open No. 2006-202586). In this method, the surface of a copper circuit plate, which is to be bonded to the silver portion of the semiconductor device, has a recessed portion which is beneficial to the large area of the surface of the copper circuit plate bonded to the silver portion of the semiconductor device.
However, in the methods disclosed in Japanese Patent Laid-Open Nos. 2011-80147, 2007-208082 and 2006-202586, if a copper plate or a copper-plated plate (an aluminum plate plated with copper) is used as a metal plate (for mounting thereon electronic parts) to bond electronic parts, such as semiconductor chips, on the metal plate with a bonding agent of a silver paste, it is not possible to bond them on such a good bonding state that the number of bonding defects is small, and it is not possible to maintain the good bonding state if heat cycles are applied thereon.